The present invention relates to an industrial method for fractionating mixtures of lipids with the aid of a solvent taken to supercritical pressure, i.e. a fluid in supercritical state or a subcritical liquid, with a view to obtaining, from used cooking oils, a refined oil, decoloured and deodorized, from which are removed products of oxidation and other products of degradation formed during use of these oils.
The consumption of fried food, particularly fried potatoes, has considerably increased in the majority of developed countries, with the development of new types of restauration and new industrially cooked foods. This has logically led to a considerable increase in the production of used frying oils. The latter represent a considerable volume of waste, amounting to hundreds of thousands of tons per year in Europe, constituting a very serious potential threat of pollution of surface water. As at the present time it cannot be envisaged to re-use them, they are generally destroyed by incineration and only their calorific power is possibly recuperated.
The present invention has for its object to propose a method for industrially treating these used cooking oils in order to obtain a refined oil capable of being used as food component for animals, particularly for poultry.
This refined oil must therefore be bereft of products likely to be detrimental to the health of the animals thus fed, and must, in addition, be obtained at a cost compatible with an industrial exploitation. Of course, this refined oil must not represent risks for the consumer of the meat of the animals thus fed either, or add unpleasant tastes to this meat. It is therefore essential to eliminate the products of degradation of the oil which are formed during cooking and in particular those which give the oil a dark, even black colour and a highly sickening characteristic odour, which characterize for example flying oils after prolonged use.
The method according to the invention will call upon a method of separation employing a fluid at supercritical pressure.
In effect, it is known that bodies are generally known in three states: solid, liquid and gaseous. One passes from one to the other by varying the temperature and/or the pressure. Now, there is a point beyond which one can pass from the liquid state to the vapour state without passing through a boiling, or, inversely, by a condensation, but continuously: this point is called the critical point.
Under these conditions, a fluid in a supercritical state is a fluid which is in a state characterized either by a pressure and temperature respectively higher than the critical pressure and temperature in the case of a pure body, or by a representative point (pressure, temperature) located beyond the envelope of the critical points represented on a diagram (pressure, temperature) in the case of a mixture. Such a fluid presents, for very numerous substances, a high solvent power incomparable to that observed for this same fluid when it is in the state of compressed gas. The same applies to so-called xe2x80x9csub-criticalxe2x80x9d liquids, i.e. which are in a state characterized either by a pressure higher than the critical pressure and by a temperature lower than the critical temperature in the case of a pure body, or by a pressure higher than the critical pressures and a temperature lower than the critical temperatures of the components in the case of a mixture.
The considerable and modulatable variations of the solvent power of these fluids are, moreover, used in numerous methods of extraction (solid/fluid), of fractionation (liquid/fluid), of analytic or preparative chromatography, of treatment of materials (ceramics, polymers). Chemical or biochemical reactions are also made in such solvents.
Among the different solvents which may be used under a supercritical pressure, carbon dioxide is particularly interesting due to its critical pressure of 7.4 MPa and its critical temperature of 31xc2x0 C., which make of it a preferred solvent in numerous applications, all the more so as it does not present any toxicity and it is available in very large quantities at very low cost. As non-polar solvent, carbon dioxide taken to supercritical pressure sometimes has a co-solvent added thereto, constituted by a polar organic solvent which modifies the solvent power in noteworthy manner especially with respect to molecules presenting a certain polarity, ethanol often being used to that end.
One of the principal advantages of the methods in which a fluid at supercritical pressure is used as solvent resides in the ease of effecting separation between the solvent and the extracts and solutes collected, as has been described in numerous publications and, for certain important aspects of implementation, in French Patent FR-A-2 584 618. The interesting properties of these fluids are moreover used in solid-fluid extraction and liquid-fluid fractionation, as is described in the afore-mentioned document.
Fractionation of the lipids by a fluid at supercritical pressure has been described in the prior state of the art, and reference may be made to a recent work xe2x80x9cSupercritical Fluid technology in Oil and Lipid Chemistryxe2x80x9d edited by J. W. KING and G. R. LIST. For example, pure carbon dioxide has been used as solvent of glycerides in order to extract the oils of different natural sources such as oleaginous seeds. The operations of fractionating lipids with the aid of solvents at supercritical pressure often encounter serious difficulties in implementation, as the initial or resultant phases are often very pasty, which renders contact with the solvent fluid difficult and even impossible to effect. Certain devices for dealing with this problem have been described, such as for example a jet extractor system proposed by EGGERS E., WAGNER H., (xe2x80x9cProceedings of the Third International Symposium on Supercritical Fluidsxe2x80x9d) in order to remove oil from a soja lecithin. However, this system comprises at the most only one theoretical stage and it is thus not possible to effect a fractionation of very similar compounds requiring a high number of theoretical plateaux generally employed on multi-staged columns with perforated plateaux or lining functioning in counter-current. This method proves to be high-performance; however, although it is well adapted to the treatment of high-quality oils of high price intended for dietetics or pharmacy, its cost price is often too high to ensure industrial purification of wastes such as cooking oils.
Another method of fractionating lipids has also been proposed, described in particular in U.S. Pat. No. 5,759,549, in which the mixture to be fractionated is adsorbed on a porous solid, from which the different components are successively extracted by a fluid at supercritical pressure whose solvent power and polarity are successively increased. This method, of which the concept has been used for several years, is known under the name of xe2x80x9cextrographyxe2x80x9d. It will be noted that this method may be carried out with a high selectivity, by combination of the selectivity of the adsorbent solid and of that of the solvent at supercritical pressure, but requires complex means and can be carried out only in batch mode, which involves very high operating costs.
The object of the present invention is to propose a method making it possible, for industrial production purposes, to effect fractionation of used cooking oils with a view to obtaining a refined oil and a residue representing a limited volume with respect to the feedstock, with the aid of a solvent taken to supercritical pressure, by using a very simple installation and inexpensive to carry out.